1. Field of the Invention
This invention relates to a method for producing a bonded wafer, which comprises steps of implanting ions of a light element such as hydrogen, helium or the like into a wafer for active layer at a predetermined depth position, bonding the wafer to a wafer for support substrate, and thereafter subjecting to a heat treatment to exfoliate the ion implanted portion as well as a bonded wafer produced by such a method.
2. Description of the Related Art
As the method for producing a bonded wafer, there is, for example, a so-called smart cut method comprising a step of implanting ions of a light element such as hydrogen, helium or the like into a wafer for active layer at a predetermined depth position to form an ion implanted layer, a step of bonding the wafer for active layer to a wafer for support substrate through an insulating film, a step of exfoliating the wafer at the ion implanted layer, and a step of thinning a portion of an active layer exposed by the exfoliation through polishing to form an active layer having a predetermined film thickness. According to this smart cut method, a portion of the wafer exfoliated after the bonding can be recycled as a wafer, which is different from the conventional bonding technique. By such a recycling can be used one wafer (wafer for active layer) in the bonded wafer plural times, which leads the way to reduce the material cost. Also, the wafer produced by the smart cut method is excellent in the uniformity of film thickness, so that the smart cut method is noticed as a production method with future potential.
However, as for the bonded wafer produced by the smart cut method, damage of about 100 nm remains in the exfoliated surface of the active layer due to the exfoliation of the ion implanted portion. Therefore, it is required to conduct a treatment for removing the damage, and there is a sacrificial oxidation as a typical technique therefor. When the sacrificial oxidation is conducted at a high temperature, however, there is caused a problem that dislocation existing in the damaged layer extends into the inside of the active layer during the heat treatment and comes to the surface as a surface defect at the subsequent step. Also, the wafer after the sacrificial oxidation is dipped in hydrofluoric acid (HF) to remove an oxide film and subjected to a reinforcement heat treatment and a planarization heat-treatment for enhancing the bonding strength, but the peeling of the active layer is caused by the subsequent planarization heat treatment (high-temperature heat treatment at 1000-1200° C. in an argon or hydrogen atmosphere) due to the scooping of the insulating film through the immersion in hydrofluoric acid and the lacking of the bonding strength, and hence there is caused a problem that defects are generated and the thickness uniformity of the active layer is broken.
Since the generation of defects and the scattering in the thickness of the active layer bring about significant bad influence on device characteristic and device performance of the wafer, it is desired to develop a method for producing the bonded wafer in which the generation of the defects can be effectively suppressed in the bonded wafer and the thickness uniformity of the active layer can be ensured in order to improve the quality of the bonded wafer.